JPH10213746A - Infrared zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain bright and high image forming performance in entire visual field and an entire zoom area by executing variable power by moving a 2nd lens group in an optical axis direction, and correcting an image forming position by moving a 3rd lens group in the optical axis direction. SOLUTION: This lens is constituted by disposing a 1st lens group I having positive refractive power, the 2nd lens group II having negative refractive power, the 3rd lens group III having the negative refractive power, a 4th lens group IV having the positive refractive power and a 5th lens group V having the positive refractive power in this order from an object side. At the time of zooming, the 1st, the 4th and the 5th lens groups I, IV and V are fixed, while the 2nd and the 3rd lens groups II and III are made movable; and the variable power is executed by moving the 2nd lens group II in the optical axis X direction and the image forming position is corrected by moving the 3rd lens group III in the optical axis X direction. The 1st to the 4th lens groups I to IV are respectively constituted of one lens L1 to L4 , and the 5th lens group V is constituted of four lenses L5 to L8 .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an infrared zoom lens,
In particular, the present invention relates to an infrared zoom lens used in an infrared optical system having a wavelength band of 3 to 5 μm or 8 to 12 μm.

[0002]

2. Description of the Related Art Conventionally, an infrared zoom lens used in an infrared optical system that scans with a mirror or the like is disclosed in, for example, U.S. Pat. Nos. 4,411,488, 4,632,498, and 4,659,
A number of proposals have been made as disclosed in the specifications of U.S. Pat. Nos. 171 and 4,676,581 and 5,022,724.

[0003]

However, conventionally,
A bright and high-performance infrared zoom lens suitable for use in a two-dimensional area sensor or the like has not been obtained. Accordingly, the present invention provides an infrared light suitable for a two-dimensional infrared area sensor or the like, which is brighter (smaller F value) and has higher image forming performance over the entire field of view and the entire zoom region as compared with a conventional infrared zoom lens. It is an object to provide a zoom lens.

[0004]

SUMMARY OF THE INVENTION An infrared zoom lens according to the present invention is an infrared zoom lens used in an infrared optical system in a wavelength band of 3 to 5 .mu.m or 8 to 12 .mu.m, and comprises one or two lenses. A first lens group having a positive refractive power, a second lens group having a negative refractive power composed of one or two lenses, and a negative meniscus having a concave surface facing the object side A third lens group composed of a lens, a fourth lens group composed of one convex lens, and a positive meniscus lens composed of at least four lenses and having a final lens on the image plane side convex toward the object side. A fifth lens group having positive refracting power is disposed in this order from the object side.
The fourth and fifth lens groups are fixed, while the second and fifth lens groups are fixed.
The third lens group is movable, and the second lens group is moved in the optical axis direction to perform zooming.
An image forming position is corrected by moving the third lens group in the optical axis direction.

In the above configuration, the following conditional expressions (1) to (1)
It is desirable to be configured to satisfy (3). _{1.00 <f 1 / f t (} 1) f 2 / f t <-0.40 (2) 0.35 <f 5 / f t <0.70 (3) However, f _t: total at the telephoto end System focal length f ₁ : Focal length of first lens group f ₂ : Focal length of second lens group f ₅ : Focal length of fifth lens group

As described above, according to the present invention, the second lens unit having negative refractive power is moved in the optical axis direction to perform zooming, and the third lens unit having negative refractive power is moved. By moving in the optical axis direction to correct the image forming position, the second and third lens groups are arranged in combination with the first and fourth lens groups having positive refractive power before and after the second and third lens groups. Thus, zoom fluctuations of coma aberration and image plane aberration (mainly, astigmatism and field curvature) can be extremely reduced. Thus, it is possible to obtain an infrared zoom lens having good imaging performance up to the periphery of the visual field and having high imaging performance over the entire zoom region.

In this case, the conditional expressions (1) to (5)
It is desirable to configure so as to satisfy (3) because
This is for the following reason.

<Lower limit value of conditional expression (1)> Below this lower limit value, the overall length becomes short, but the occurrence of spherical aberration and coma becomes large, and it is possible to obtain a zoom lens which is bright and has good imaging performance. Can not. In addition, the fluctuation of the image plane aberration due to the zoom increases, and the zoom ratio cannot be increased.

If the upper limit of conditional expression (2) is exceeded, the spherical aberration, coma aberration, and image plane aberration greatly fluctuate due to zooming, and a bright zoom lens having a large zoom ratio cannot be obtained.

<Lower limit value of conditional expression (3)> Below this lower limit value, the overall length becomes short, but the spherical aberration, coma aberration, and image plane aberration increase greatly due to zooming, and the zoom lens is bright and has a large zoom ratio. Can not get. In addition, there arises a problem that a necessary back focus cannot be obtained.

<Upper limit of conditional expression (3)> Above this upper limit, the overall length becomes longer, and the lens diameter and the aperture diameter of the fifth lens unit become large, so that it is impossible to make the fifth lens unit compact.

[0012]

Embodiments of the present invention will be described below. <First lens group> One convex lens made of a material having a small dispersion,
Alternatively, it is composed of two lenses of two kinds of materials having different dispersions. In particular, in the latter case, chromatic aberration of magnification can be satisfactorily corrected, and high optical performance can be obtained with a long focal length zoom lens or a lens with a large zoom ratio.

For a lens in the 3 to 5 μm band, it is most preferable to form a two-concave lens made of silicon and germanium. However, since the material for infrared rays is expensive, the large-diameter lens on the front side can be made of one piece of silicon with small dispersion. Further, in a lens in the 8 to 12 μm band, germanium has a very small dispersion, so that germanium 1
A single configuration is possible. The first lens group can have a focusing function by moving in the optical axis direction.

<Second lens group> A second lens group is composed of one concave lens made of a material having a small dispersion or two lenses made of two materials having different dispersions. In particular, with the latter configuration, longitudinal chromatic aberration can be favorably corrected, and high optical performance can be obtained in a long focal length zoom lens. 3-5μ
It is most preferable that the lens in the m band be constituted by two convex and concave lenses of silicon and germanium. However, in the case of a short focus zoom lens or a zoom lens having a relatively small zoom ratio, it is possible to use a single piece of silicon having a small dispersion.

For a lens in the 8 to 12 μm band, it is most preferable to use two lenses of zinc selenide and germanium. However, in a lens in the 8 to 12 μm band, the dispersion of germanium is very small, so that a single germanium structure is possible.

<Third lens group> 1 having a concave surface facing the object side
By using a negative meniscus lens, it is possible to reduce fluctuations in spherical aberration, coma aberration, and image plane aberration due to zooming.

<Fourth lens group> The fourth lens group is composed of one convex lens and has a function of making the light beams from the zoom units of the first to third lens groups substantially afocal. By arranging the stop between the fourth lens group and the fifth lens group, it is possible to secure 100% of the light amount around the visual field.

<Fifth lens group> A convex lens group composed of at least two lenses having an action of correcting spherical aberration and axial chromatic aberration is arranged at the front, and at least two lens groups are arranged at the rear. A positive meniscus lens having a convex surface facing the object side is disposed on the final lens on the image side, and coma aberration, astigmatism, and field curvature are corrected.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments 1 to 5 of the present invention will be described below with reference to the drawings. <Embodiment 1> FIG. 1 is a diagram showing a lens configuration at the wide-angle end, showing an infrared zoom lens according to the present embodiment.
FIG. 3 is a diagram showing a positional relationship of each lens unit at a wide-angle end (a), an intermediate position (b), and a telephoto end (c).

As shown in FIG. 1, the infrared zoom lens according to the present embodiment has a first lens unit I having a positive refractive power.
A second lens group II having a negative refractive power, a third lens group III having a negative refractive power, and a fourth lens group III having a positive refractive power.
A lens group IV and a fifth lens group V having a positive refractive power are arranged in this order from the object side. During zooming, the first, fourth, and fifth lens groups I, IV, V While being fixed, the second and third lens groups II and III are made movable. By moving the second lens group II in the optical axis X direction, zooming is performed, and Lens group II
The imaging position is corrected by moving I in the optical axis X direction. Further, the infrared zoom lens is configured to satisfy the following conditional expressions (1) to (3).

1.00 <f ₁ / f _t (1) f ₂ / f _t <−0.40 (2) 0.35 <f ₅ / f _t <0.70 (3) where f _t : telephoto The focal length of the entire system at the end f ₁ : the focal length of the first lens group f ₂ : the focal length of the second lens group f ₅ : the focal length of the fifth lens group The same applies to an infrared zoom lens.

In this embodiment, the first, second, third and fourth lens units I, II, III and IV are composed of one lens L ₁ , L _1
2, L _3, L ₄ are each composed of a fifth lens group V
It is composed of four lenses L ₅ ~L _8.

Here, the first lens L ₁ , the fifth lens L ₅ ,
The eighth lens L ₈ is a positive meniscus lens having a convex surface facing the object side, the fourth lens L ₄ , the seventh lens L ₇ is a positive meniscus lens having a convex surface facing the image surface, and the second lens L
Reference numeral ₂ denotes a negative meniscus lens having a concave surface facing the image surface side, and the third lens L ₃ and the sixth lens L ₆ are negative meniscus lenses having a concave surface facing the object side. Between the fifth lens group V and the image plane 1, the dewar window (window detector) L ₉ are arranged.

Table 1 shows the radius of curvature r (mm) of each lens surface of the infrared zoom lens according to the present embodiment, the center thickness of each lens, and the air space between the lenses (hereinafter, these are collectively referred to as axial upper surface space). D) (mm), and the materials constituting each lens. The numbers in Table 1 indicate the order from the object side. Table 2 shows the values of D ₁ , D ₂ , and D ₃ at the wide-angle end, the intermediate position, and the telephoto end in the column of the shaft upper surface distance d in Table 1. Furthermore, this Table 2
The lower part shows the focal length f, F number (F _NO ), and wavelength band (λ) of the entire system.

[0025]

[Table 1]

[0026]

[Table 2]

3 to 5 are aberration diagrams showing various aberrations of the infrared zoom lens according to the present embodiment. FIG.
FIG. 4 shows various aberrations at the intermediate position, and FIG. 5 shows various aberrations at the telephoto end. As is apparent from these figures, according to the present embodiment, in the wavelength band of 8 to 12 μm, the infrared zoom having good imaging performance up to the periphery of the visual field and having high imaging performance over the entire zoom region. You can get a lens.

<Embodiment 2> FIG. 6 is a diagram showing a configuration of an infrared zoom lens according to the present embodiment at the wide-angle end. FIG. 7 shows the wide-angle end (a), the intermediate position (b), and the telephoto end. FIG. 3C is a diagram illustrating a positional relationship between the lens groups in FIG.

As shown in FIG. 6, the infrared zoom lens according to the present embodiment includes first, third and fourth lens groups I, III,
IV is composed of one lens L ₁ , L ₄ , L ₅ , the second lens group II is composed of _two lenses L ₂ , L ₃ , and the fifth lens group V is And four lenses L _{6 to} L ₉ .

Here, the first lens L ₁ , the sixth lens L ₆ ,
The ninth lens L ₉ is a positive meniscus lens having a convex surface facing the object side, the fifth lens L ₅ and the eighth lens L ₈ are positive meniscus lenses having a convex surface facing the image surface, and the second lens L
₂ is a negative meniscus lens having a concave surface facing the image surface side, the fourth lens L ₄ and the seventh lens L ₇ are negative meniscus lenses having a concave surface facing the object side, and the third lens L ₃ is This is a biconcave lens whose surface has a strong curvature. Fifth lens group V
And between the image plane 1, L ₁₀ (window detector) dewar window is arranged.

Table 3 shows the radius of curvature r (mm) of each lens surface, the center thickness of each lens, the air gap d (mm) between each lens, and each lens of the infrared zoom lens according to this embodiment. Show the material. Further, in Table 4, the D _1, D _2, D ₃ in the column axial distance d in Table 3, the wide-angle end,
The values at the intermediate position and the telephoto end are shown. Further, in the lower part of Table 4, the focal length f of the entire system, F number (F _NO ),
The wavelength band (λ) is shown.

[0032]

[Table 3]

[0033]

[Table 4]

8 to 10 are aberration diagrams showing various aberrations of the infrared zoom lens according to the present embodiment. FIG. 8 shows various aberrations at the wide-angle end, FIG. 9 shows various aberrations at the intermediate position, and FIG. ing. As is apparent from these figures, according to the present embodiment, in the wavelength band of 8 to 12 μm, the infrared zoom having good imaging performance up to the periphery of the visual field and having high imaging performance over the entire zoom region. You can get a lens.

<Embodiment 3> FIG. 11 is a diagram showing the configuration of an infrared zoom lens according to this embodiment at the wide-angle end, and FIG. 12 shows the wide-angle end (a), the intermediate position (b), and the telephoto end. FIG. 3C is a diagram illustrating a positional relationship between the lens groups in FIG.

As shown in FIG. 11, in the infrared zoom lens according to the present embodiment, the first lens group I is composed of _two lenses L ₁ and L ₂ , and the second lens group II is also composed of two lenses. lenses L _3, is composed of L _4, third and fourth lens groups III, IV is respectively composed of one lens L _5, L _6, the fifth lens group V are 6 Pieces of lens L _{7 to} L ₁₂
It is composed of

Here, the first lens L ₁ , the fourth lens L ₄ ,
The seventh lens L ₇ and the twelfth lens L ₁₂ are a positive meniscus lens having a convex surface facing the object side, the sixth lens L ₆ , and the eleventh lens L 12.
Lens L _{1 1} is a positive meniscus lens having a convex surface directed toward the image side, the third lens L _3, the eighth lens L _8, a negative meniscus lens having a concave surface on the image side, a fifth lens L ₅ Is a negative meniscus lens having a concave surface facing the object side, the second lens L ₂ is a plano-concave lens having a concave surface facing the image surface side, and the ninth lens L ₉ is a surface having a strong curvature facing the image surface side. biconcave lens, a tenth lens L ₁₀ is a biconcave lens having a surface with a stronger curvature on the object side. Between the fifth lens group V and the image plane 1, the band-pass filter L ₁₃ and (window detectors) Dewar window L ₁₄ selects the wavelength band it is arranged.

Table 5 shows the radius of curvature r (mm) of each lens surface of the infrared zoom lens according to this embodiment, the center thickness of each lens, the air gap d (mm) between each lens, and each lens. Show the material. In Table 6, D ₁ , D ₂ , D ₃ in the column of the shaft top distance d in Table 5, the wide-angle end,
The values at the intermediate position and the telephoto end are shown. Further, in the lower part of Table 6, the focal length f, F number (F _NO ),
The wavelength band (λ) is shown.

[0039]

[Table 5]

[0040]

[Table 6]

FIGS. 13 to 15 are aberration diagrams showing various aberrations of the infrared zoom lens according to this embodiment. FIG. 13 shows various aberrations at the wide-angle end, FIG. 14 shows various aberrations at the intermediate position, and FIG. ing. As is apparent from these figures, according to the present embodiment, in the wavelength band of 3 to 5 μm, the infrared zoom having good imaging performance up to the periphery of the visual field and having high imaging performance over the entire zoom region. You can get a lens.

<Embodiment 4> FIG. 16 is a diagram showing the configuration of an infrared zoom lens according to the present embodiment at the wide-angle end. FIG. 17 shows the wide-angle end (a), the intermediate position (b), and the telephoto end. FIG. 3C is a diagram illustrating a positional relationship between the lens groups in FIG.

As shown in FIG. 16, in the infrared zoom lens according to the present embodiment, the first lens group I is composed of _two lenses L ₁ and L ₂ , and the second lens group II is also composed of two lenses. lenses L _3, is composed of L _4, third and fourth lens groups III, IV is respectively composed of one lens L _5, L _6, the fifth lens group V are 5 The lenses L _{7 to} L ₁₁
It is composed of

Here, the first lens L ₁ , the fourth lens L ₄ ,
The seventh lens L ₇ and the eleventh lens L ₁₁ are a positive meniscus lens having a convex surface facing the object side, the sixth lens L ₆ ,
Lens L ₁₀ is a positive meniscus lens having a convex surface directed toward the image side, the third lens L _3, the eighth lens L _8, a negative meniscus lens having a concave surface on the image side, a fifth lens L ₅ is ,
A negative meniscus lens having a concave surface facing the object side, a second lens L ₂ is a plano-concave lens having a concave surface facing the image surface side, and a ninth lens L ₉ is a biconcave lens having a surface of strong curvature facing the image surface side It is. Between the fifth lens group V and the image plane 1, a band-pass filter L ₁₂ and dewar windows (detector window) L ₁₃ selects the wavelength band it is arranged.

Table 7 shows the radius of curvature r (mm) of each lens surface of the infrared zoom lens according to this embodiment, the center thickness of each lens, the air gap d (mm) between each lens, and each lens. Show the material. In Table 8, D ₁ , D ₂ , and D ₃ in the column of the shaft upper surface distance d in Table 7 indicate the wide-angle end,
The values at the intermediate position and the telephoto end are shown. Further, in the lower part of Table 8, the focal length f of the entire system, F number (F _NO ),
The wavelength band (λ) is shown.

[0046]

[Table 7]

[0047]

[Table 8]

18 to 20 are aberration diagrams showing various aberrations of the infrared zoom lens according to this embodiment. FIG. 18 shows various aberrations at the wide angle end, FIG. 19 shows various aberrations at the intermediate position, and FIG. ing. As is apparent from these figures, according to the present embodiment, in the wavelength band of 3 to 5 μm, the infrared zoom having good imaging performance up to the periphery of the visual field and having high imaging performance over the entire zoom region. You can get a lens.

<Embodiment 5> FIG. 21 is a diagram showing the configuration of an infrared zoom lens according to this embodiment at the wide-angle end. FIG. 22 shows the wide-angle end (a), the intermediate position (b), and the telephoto end. FIG. 3C is a diagram illustrating a positional relationship between the lens groups in FIG. As shown in FIG. 21, the infrared zoom lens according to this example includes first, second, third, and fourth lens groups I, II, and I.
II, IV is a single lens L _1, L _2, L _3, are each composed of L _4, a fifth lens group V are five lenses L ₅ ~L ₉
It is composed of

Here, the first lens L₁, The fifth lens L_Five,
9th lens L₉Is a positive menisca with the convex surface facing the object side
Lens, fourth lens L_Four, The eighth lens L₈On the image side
Positive meniscus lens with convex surface, sixth lens L
₆Is a negative meniscus lens with the concave surface facing the image side,
3 lenses L_Three, The seventh lens L₇Has a concave surface facing the object side
Negative meniscus lens, second lens L_TwoIs strong on the image side
It is a biconcave lens whose surface has a large curvature. Fifth lens group V
A bandpass filter for selecting a wavelength range
Ruta L _TenAnd dewar window (detector window) L₁₁Is placed
Have been.

Table 9 shows the radius of curvature r (mm) of each lens surface, the center thickness of each lens, the air gap d (mm) between each lens, and each lens of the infrared zoom lens according to this embodiment. Show the material. Table 10 shows the values of D ₁ , D ₂ , and D ₃ at the wide-angle end, the intermediate position, and the telephoto end in the column of the shaft top distance d in Table 9. Further, in the lower part of Table 10, the focal length f and the F number (F
_NO ) and the wavelength band (λ).

[0052]

[Table 9]

[0053]

[Table 10]

23 to 25 are aberration diagrams showing various aberrations of the infrared zoom lens according to this embodiment. FIG. 23 shows various aberrations at the wide-angle end, FIG. 24 shows various aberrations at the intermediate position, and FIG. ing. As is apparent from these figures, according to the present embodiment, in the wavelength band of 3 to 5 μm, the infrared zoom having good imaging performance up to the periphery of the visual field and having high imaging performance over the entire zoom region. You can get a lens.

[0055]

As described above, according to the present invention,
It is possible to obtain an infrared zoom lens that is brighter (has a smaller F-number) and has higher imaging performance over the entire field of view and the entire zoom region, and is suitable for a two-dimensional infrared area sensor, etc., as compared with a conventional infrared zoom lens. it can.

[Brief description of the drawings]

FIG. 1 is a lens configuration diagram at a wide-angle end showing an infrared zoom lens according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a positional relationship of each lens group of the infrared zoom lens according to the first embodiment at a wide-angle end (a), an intermediate position (b), and a telephoto end (c).

FIG. 3 is an aberration diagram showing various aberrations of the infrared zoom lens according to Example 1 at a wide-angle end.

FIG. 4 is an aberration diagram showing various aberrations of the infrared zoom lens according to Example 1 at an intermediate position.

FIG. 5 is an aberration diagram showing various aberrations at the telephoto end of the infrared zoom lens according to Example 1;

FIG. 6 is a lens configuration diagram at the wide-angle end, showing an infrared zoom lens according to Embodiment 2 of the present invention.

FIG. 7 is a diagram showing a positional relationship of each lens group of the infrared zoom lens according to Example 2 at a wide-angle end (a), an intermediate position (b), and a telephoto end (c).

FIG. 8 is an aberration diagram showing various aberrations at the wide-angle end of the infrared zoom lens according to Example 2;

FIG. 9 is an aberration diagram showing various aberrations of the infrared zoom lens according to Example 2 at an intermediate position.

FIG. 10 illustrates an infrared zoom lens according to the second embodiment.
Aberration diagram showing various aberrations at the telephoto end

FIG. 11 is a lens configuration diagram at a wide-angle end, showing an infrared zoom lens according to a third embodiment of the present invention.

FIG. 12 shows the infrared zoom lens according to Example 3;
FIG. 4 is a diagram showing a positional relationship of each lens group at a wide-angle end (a), an intermediate position (b), and a telephoto end (c).

FIG. 13 illustrates an infrared zoom lens according to Example 3;
Aberration diagram showing various aberrations at the wide-angle end

FIG. 14 shows the infrared zoom lens according to Example 3;
Aberration diagram showing various aberrations at the intermediate position

FIG. 15 shows the infrared zoom lens according to Example 3;
Aberration diagram showing various aberrations at the telephoto end

FIG. 16 is a lens configuration diagram at the wide-angle end showing an infrared zoom lens according to Embodiment 4 of the present invention.

FIG. 17 illustrates an infrared zoom lens according to Example 4;
FIG. 4 is a diagram showing a positional relationship of each lens group at a wide-angle end (a), an intermediate position (b), and a telephoto end (c).

FIG. 18 illustrates an infrared zoom lens according to the fourth embodiment.
Aberration diagram showing various aberrations at the wide-angle end

FIG. 19 illustrates an infrared zoom lens according to the fourth embodiment.
Aberration diagram showing various aberrations at the intermediate position

FIG. 20 illustrates an infrared zoom lens according to Example 4;
Aberration diagram showing various aberrations at the telephoto end

FIG. 21 is a diagram showing a lens configuration at a wide-angle end, showing an infrared zoom lens according to Embodiment 5 of the present invention.

FIG. 22 illustrates an infrared zoom lens according to Example 5;
FIG. 4 is a diagram showing a positional relationship of each lens group at a wide-angle end (a), an intermediate position (b), and a telephoto end (c).

FIG. 23 illustrates the infrared zoom lens according to Example 5;
Aberration diagram showing various aberrations at the wide-angle end

FIG. 24 shows the infrared zoom lens according to Example 5;
Aberration diagram showing various aberrations at the intermediate position

FIG. 25 illustrates an infrared zoom lens according to Example 5;
Aberration diagram showing various aberrations at the telephoto end

[Explanation of symbols]

 L Lens and dewar window r Radius of curvature d Axis upper surface distance X Optical axis 1 Image plane I First lens group II Second lens group III Third lens group IV Fourth lens group V Fifth lens group

[Table 11]

Claims (2)

[Claims] 1. An infrared zoom lens used in an infrared optical system in a wavelength band of 3 to 5 μm or 8 to 12 μm, wherein the first lens having one or two lenses and having a positive refractive power. Group, a second lens group having a negative refractive power composed of one or two lenses, a third lens group including one negative meniscus lens having a concave surface facing the object side,
A fourth lens group including one convex lens, and a fourth lens group including at least four lenses and having a positive refractive power including a positive meniscus lens in which the final lens on the image side has a convex surface facing the object side. Five lens groups are arranged in this order from the object side. During zooming, the first, fourth, and fifth lens groups are fixed, while the second and third lens groups are movable. The zooming is performed by moving the second lens group in the optical axis direction, and the imaging position is corrected by moving the third lens group in the optical axis direction. Features infrared zoom lens. 2. The infrared zoom lens according to claim 1, wherein the zoom lens is configured to satisfy the following conditional expressions (1) to (3). _{1.00 <f 1 / f t (} 1) f 2 / f t <-0.40 (2) 0.35 <f 5 / f t <0.70 (3) However, f _t: total at the telephoto end System focal length f ₁ : Focal length of first lens group f ₂ : Focal length of second lens group f ₅ : Focal length of fifth lens group